The suitable selection of the membrane based on porosity allows its effective industrial application such as gas separation, nanofiltration, ultrafiltration, prefiltration, clarification, acid separation etc. The separation performance, especially flux and selectivity are a function of the nature of the membranes. To increase the permeability of the membrane, asymmetric membranes were developed that overcomes the disadvantages of the symmetric membrane
The characteristic feature of asymmetric membranes is the presence of a thin, selective top layer (skin) and a porous sublayer with large through pores, which minimizes the mass transfer resistance of membranes. Asymmetric membranes can have either a nonporous or a porous skin. The latter membranes can be used as supports in the composite membranes or as independent porous films in various pressure-driven processes (reverse osmosis, microfiltration, ultrafiltration, and nanofiltration).
Morphology and the transport properties of asymmetric membrane can be controlled by the selection of polymer, in the process of preparation, the preparation conditions, the solvents used and other such parameters. It has been well documented in the literature that water soluble macromolecules adsorb readily on the polymeric ultrafiltration membranes [Ultrafiltration Membranes and Applications, Polym. Sci. Tech., 13, 1981, pp. 141-158].
In most known cases, asymmetric membranes are cast from solutions of polymers based on homologous series of cellulose esters, aromatic polyimides, or polysulfones, polyolefins, etc.
An article titled “Separation performance of asymmetric membranes based on PEGDa/PEI semi-interpenetrating polymer network in pure and binary gas mixtures of CO2, N2 and CH4” by Sundar Saimani, Mauro M. Dal-Cin et al in Journal of Membrane Science, Volume 362, Issues 1-2, Pages 353-359, 2010 relate to asymmetric membranes of semi-interpenetrating polymer networks (semi-IPN) prepared with commercial poly (ether imide) (ULTEM®) and poly (ethylene glycol) diacrylate (PEGDa) in 1-methyl-2-pyrrolidinone (NMP).
Article titled “Preparation and characterization of highly selective dense and hollow fiber asymmetric membranes based on BTDA-TDI/MDI co-polyimide” by J. N. Barsema, G. C. Kapantaidakis et. al in Journal of Membrane Science, Volume 216, Issues 1-2, Pages 195-205, 2003 report the preparation, characterization, and the permeation properties of dense flat sheet and asymmetric hollow fiber membranes, based on BTDA-TDI/MDI co-polyimide.
Polybenzimidazole (PBI) to be used as a membrane material is attracting considerable attention due to its excellent thermochemical and mechanical stability.
US 2012/0000852 discloses membrane comprising polybenzimidazole and aromatic polyester wherein said aromatic polyester is removed. The membrane is used for a process of deacidification.
U.S. Pat. No. 6,623,639 discloses method of making a polybenzimidazole microporous hollow fiber membrane in presence of polyvinyl pyrollidinone and n-propanol. U.S. Pat. No. 4,842,740 discloses membranes produced from the blend of polybenzimidazole polymers and polyarylate polymers.
Further fabrication of polybenzimidazole (PBI) nano-filtration hollow fiber membranes for removal of chromate from wastewater is disclosed in Journal of Membrane Science Vol 281, Issues 1-2, 15 Sep. 2006, Pages 307-315 by Kai Yu Wang et al. Ulhas K. Kharul et al. in European Polymer Journal Volume 45, Issue 12, December 2009, Pages 3363-3371 discloses synthesis of series of N-substituted Polybenzimidazoles (PBI) using selective alkyl groups with varying bulk and flexibility, viz., methyl, n-butyl, methylene trimethylsilane and 4-tert-butylbenzyl. PBI-I based on 3,3′-diaminobenzidine (DAB) and isophthalic acid and PBI-BuI based on DAB and 5-tert-butyl isophthalic acid were chosen for N-substitution.
Further Ulhas K. Kharul et al in Journal of Membrane Science 286 (2006) 161-169 discloses enhancement of gas permeation properties of polybenzimidazoles by systematic structure architecture wherein polybenzimidazoles based on 3,3-diaminobenzidene and 5-tert-butyl isophthalic acid or 4,4-(hexafluoroisopropylidene) bis(benzoic acid) exhibited excellent combination of physical and gas permeation properties as compared to the PBI based on isophthalic acid.
The determination of rejection/molecular weight cut off (MWCO) is one of the popular methods to characterize the membrane pore size.
The MWCO is obtained by plotting rejection of selected solutes versus their molar mass, where retention of PEGs in cross-flow ultrafiltration through membranes is reported in Desalination Volume 149, Issues 1-3, 10 Sep. 2002, Pages 417-422 by Samantha Platt.
The polymers used to cast membrane in the art have various limitations. For e.g. Polyimides are not known to operate under more severe temperatures or in corrosive media, in particular, those containing organic solvents, are costly and have limited solubility.
The present inventors have observed that it is mainly the choice of the polymer that predetermines the variety of problems to be tackled such as the physicochemical properties that are inherent in a given polymer that imposes limitations on the choice of solvents and non-solvents and is manifested in the morphology of the corresponding membrane.
In view of the above, BuPBI (tert-butyl-polybenzimidazole), a versatile material of thermo-chemically and mechanically stable PBI family appears to hold more promise in the synthesis of asymmetric membranes whose porosity can be tuned as per the required separation application. Further tert-butyl group in these PBI led to the lowering in packing density, a small reduction in thermal stability and enhanced solvent solubility. While analysing rejection performance, PBI-BuI based membrane was found to exhibit interactions even with the neutral PEG molecules.